Symmetry vs. asymmetry of a hyper-thinned rift: Example of the Mauléon Basin (Western Pyrenees, France)

Abstract

Abstract The aim of this study is to unravel the tectono-sedimentary evolution of a hyper-thinned rift, based on the example of the Mauleon Basin, a basin filled by thick synrift deposits. The integrated study combines field data, detailed geological mapping and seismic interpretation. The field study focuses on the Iberian margin of the Mauleon Basin. Seismic interpretation and well calibration along a N S transect of the Mauleon Basin enable imaging the transition with the northern conjugate margin. The synrift records are very different on either side of the basin: the southern margin is composed of a proximal turbiditic s.l. siliciclastic system, whereas the northern margin is characterized by a carbonate system extending from the platform to the basin. We recognize the Mauleon rift as an apparent symmetric hyper-thinned rift, related to a southward dipping Albian detachment and a northward dipping Cenomanian one. Two stages of continental crustal thinning are inferred to explain the development of the Mauleon Basin. First, a Barremian to earliest Albian “ductile pure-shear thinning phase”, responsible for the lower crustal thinning and the formation of a symmetric sag basin. Second, an Albian-Cenomanian simple-shear thinning phase, responsible for the onset of the southward dipping Saint-Palais detachment faulting and for evolution to an asymmetric basin. The Iberian margin appears as an upper plate and the European one as a lower plate during Albian time. At Early Cenomanian time, the basin was affected by structural changes of the margins resulting from shift in detachment direction, interpreted as “flip-flop detachment tectonics.